Contact Lens Vertex Distance Calculator

Vertex distance is the distance between the back surface of your glasses lens and the front of your cornea, measured in millimeters. The standard value used in contact lens vertex conversion is 12 mm. Because a contact lens sits directly on the eye (vertex distance of 0 mm) while glasses sit 10–14 mm in front of it, the same lens power produces a different effective power, and the vertex formula compensates for that shift.

Use the back vertex formula Fc = F / (1 − d·F), where F is the glasses prescription in diopters and d is the vertex distance in meters. For example, with a −6.00 D glasses prescription and a 12 mm (0.012 m) vertex distance: Fc = −6.00 / (1 − 0.012·(−6.00)) = −6.00 / 1.072 ≈ −5.60 D, which rounds to −5.50 D — the standard contact lens power. Enter your numbers in the calculator above to get the exact conversion.

The back vertex formula is Fc = F / (1 − d·F). Fc is the equivalent contact lens power, F is the spectacle (glasses) lens power in diopters, and d is the vertex distance in meters. It computes the back vertex power of a lens combination, giving the lens power at the cornea that is optically equivalent to the glasses lens at the original vertex distance.

They differ because of vertex distance. Glasses sit 10–14 mm away from your eyes, while contact lenses sit directly on the cornea. Moving a lens closer to the eye reduces the effective power of minus (nearsighted) lenses and increases the effective power of plus (farsighted) lenses. That is why a −6.00 D glasses prescription becomes roughly −5.50 D in contacts, while a +5.00 D prescription becomes about +5.25 D.

Vertex distance becomes clinically significant at about ±4.00 D. Below that power, the difference between glasses and contact lens power is typically less than 0.12 D — smaller than the 0.25 D steps contact lenses are made in, so the power is usually identical. Above ±4.00 D the difference grows quickly: a −8.00 D glasses prescription converts to about −7.25 D in contacts, a shift of 0.75 D that is clearly noticeable.

Minus (myopia/nearsighted) lenses get weaker in contacts — the contact lens power is numerically smaller than the glasses power. For example, −8.00 D glasses convert to approximately −7.25 D in contacts at a 12 mm vertex distance. The reverse is true for plus lenses: a +8.00 D glasses prescription converts to about +8.75 D in contacts.

Contact lenses are manufactured in 0.25 D steps for most powers. Round the calculated result to the nearest 0.25 D: −5.60 D becomes −5.50 D, and +5.32 D becomes +5.25 D. This calculator does that rounding for you automatically, showing both the exact computed value and the rounded practical value.

12 mm is the standard default vertex distance used by optometrists for contact lens conversions when the actual distance was not measured. If your optician measured a different vertex distance (for example 10 mm or 14 mm for high-minus frames that sit farther forward), enter that value instead. Larger vertex distances produce larger power differences in the conversion.

Vertex distance compensation applies to the full lens power including cylinder, but the conversion is usually applied to the spherical equivalent (sphere + cylinder/2) rather than to each meridian separately. Cylinder values under about 1.50 D usually convert directly to toric contact lens steps; higher cylinders may be compensated meridian by meridian by the optometrist. This calculator handles the sphere power conversion.

No. This calculator is an educational estimate, not a prescription. A real contact lens fitting also requires base curve, diameter, lens material, tear film assessment, and a trial lens evaluation by an optometrist or ophthalmologist. Always get your actual contact lens prescription from an eye care professional before ordering lenses.

A vertex conversion chart lists glasses powers in the left column and the matching contact lens powers next to them for a fixed vertex distance (usually 12 mm). Find your glasses power in the left column and read across to the contact lens column. The reference chart on this page covers −12.00 D to +12.00 D, so you can look up common values like −6.00 → −5.50 D or +6.00 → +6.50 D at a glance.

With F = +6.00 D and d = 0.012 m: Fc = 6.00 / (1 − 0.072) = 6.47 D, which rounds to +6.50 D — stronger than the glasses. This is because a plus lens moved closer to the eye must bend light more to converge at the same retina.

Back vertex power (BVP) is the effective power measured from the back surface of the lens — it is the standard labeled power of any ophthalmic lens and what the vertex formula compensates. Front vertex power is measured from the front surface and matters mainly in thick high-power lenses. For contact lens conversion, always use the back vertex power printed on your glasses prescription.

The formula applies to any spherical lens power, including the reading (near) portion of a prescription. For bifocal or multifocal wearers switching to multifocal contact lenses, the distance power is converted with the vertex formula and the near addition (add power) is handled separately by the multifocal lens design. Add powers do not get vertex-compensated the same way.

The vertex compensation math is the same for gas permeable (RGP) and soft lenses because it depends only on glasses power and vertex distance, not lens material. However, RGP fitting can include additional power adjustments: the tear lens under a rigid lens has refractive power (about ±0.25 D) that the optometrist accounts for, and high-cylinder RGP fits use different compensation strategies.

The calculator is mathematically exact for the standard back vertex formula Fc = F / (1 − d·F) — the same formula used in clinical optometry textbooks and lensometry. Every result on this page can be verified by hand with that equation. Its limitation is clinical, not mathematical: it cannot measure your actual vertex distance, base curve, or eye health, so treat results as an estimate and confirm with an optometrist.